Generally, a lower wire guide device positioned inside a work tank of a wire electric discharge machine is supported at the end of a horizontally extending arm. The arm is fixed to a side surface of a column, for example, at a base end of the arm, and passes through an elongated hole formed in one of the work tank walls. An opening of the elongated hole normally extends in the X-axis direction and the arm extends in the Y-axis direction so that the work tank can move within the X-Y plane relative to the arm. The elongated hole is sealed in a fluid tight manner by a slide plate having a hole through which the arm passes. If the arm is long, there are large variations in the position of the lower wire guide device due to bending in a direction perpendicular to the arm.
U.S. Pat. No. 5,897,791 discloses a wire electric discharge machine having a mechanism for driving a lower wire guide device positioned inside a work tank within the X-Y plane. With this type of wire electric discharge machine, there is no need for an arm for attaching the lower wire guide mechanism, which obviously means that there is no need to seal an arm. A unit including a servo motor, ball screw, guide rail, position sensor, etc. is positioned inside a work tank filled with dielectric fluid, so they are covered in a fluid-tight manner by bellows. However, this unit is heavier than the conventional arm, making it necessary for a mechanism supporting the unit to be sufficiently large. Great care must be taken with respect to protecting the drive mechanism from invasion by dielectric fluid.